1. Field of the Invention
The present invention relates to a continuous hot strip rolling system and a method thereof More particularly, the present invention pertains to a continuous hot strip rolling system in which bars are joined to each other between a roughing train and a finishing train for continuous rolling, and a method thereof.
Regarding the hot strip rolling system, there has been a strong demand for sequentially joining the materials to be rolled to conduct continuous rolling in the finishing train, and various proposals have been made to meet this demand. However, none of these proposals has been put into practical use. Joining of the materials to be rolled for continuous rolling (hereinafter referred to as a "joining continuous rolling") has been desired because it can improve feeding of thin strips, because it enables a shape control function utilizing tension rolling to be provided, because it enables a high reduction rolling to be conducted in a subsequent stand due to supply of a lubricant, and because it enables strip curving camber to be reduced. Generation of camber causes troubles not only in the finish rolling process but also in the subsequent processes, such as a pickling process or cold rolling process When strips have camber, the leading and trailing ends of that strip must be cut off, thus greatly reducing yield
In most of the conventional joining continuous rolling techniques, the bars are joined to each other between the roughing and finishing trains, as in the case of, for example, U.S. Pat. No. 4,706,871. In U.S. Pat. No. 4,706,871, all the mill stands, including those in the roughing train, are shown in FIGS. 1 and 2 as if they were two-high mill stands, for the purpose of simplifying illustration thereof. However, it has actually not been practiced that all the mill stands in the roughing train be constituted of the two-high mill stands alone. In the case of unidirectional rolling, four-high mill stands are generally employed. These four-high mill stands are disposed separately at a distance from each other, which distance increases as the material being rolled becomes longer due to rolling so that the same material being rolled is not caught by two adjacent mill stands at the same time, as in the case of U.S. Pat. No. 4,706,871. This arrangement of the four-high mill stands is advantageous, because it allows individual mill stand to be driven independently and thus allows inexpensive motors, such as a synchronous motor which does not require speed control, to be used. In the rolling technique disclosed in JP, A, 58-112601, a single reversable roughing mill stand for conducting the unidirectional rolling is provided in place of the roughing train.
In U.S. Pat. No. 4,444,038 which is not concerned with the joining continuous rolling technique, all the mill stands are shown as being the two-high mill stands for the purpose of simplifying illustration, as in the case involving the aforementioned U.S. Pat. No. 4,706,871 Meanwhile, the use of two-high mill has been proposed in the conventional techniques disclosed in, for example, JP, A, 2-235502, JP, A, 61-17305, JP, A, 61-56708, JP, A, 50-95160, and JP, A, 50-109866. In JP, A, 2-235502, it has been proposed to alternately use three pairs of rolls in order to prevent overheating of the mill and to incorporate two pairs of upstream rolls in a common housing. JP, A, 61-17305 and JP, A, 61-56708 have disclosed the use of a common housing for a two-high mill and a four-high mill to achieve high reduction rolling. JP, A, 50-109866 has proposed the incorporation of both a planetary mill and two sets of two-high mills in a common housing.
In the conventional joining continuous rolling techniques, roughly rolled bars having a thickness from 30 to 40 mm are joined to each other. In order to reduce a temperature difference between leading and trailing ends of the bars to be joined, it has been proposed in JP, A, 58-112601 to coil each bar at the exit of the roughing train and then to uncoil it for joining.
In the hot rolling process, when the material being rolled has a scaled surface, the scale bites into the surface of a product, leaving a flaw and thus greatly deteriorating the quality of the product. Hence, a descaling device for ejecting water under pressure is provided on the rolling line for peeling off or removing the scale on the surface of the material to be rolled. Although the water ejecting nozzle of such a descaling device is generally of the fixed type, JP, A, 63-68213 discloses a descaling device having a pivotal nozzle to improve the scale removal performance.
In the aforementioned JP, A, 50-95160 which discloses a rolling system in which a planetary mill and two sets of two-high mills are disposed close to each other, a gas device is provided to cover the portions of the material being rolled between the planetary mill and the two-high mill located adjacent to the planetray mill and between the two two-high mills, with an inactive or reducing gas in order to prevent generation of scale.
However, the conventional joining continuous rolling systems have the following drawbacks.
In the roughing train, when the bar is roughly rolled to a thickness of about 30 to 40 mm by the four-high mill stands, generation of camber cannot be avoided. Such a bar is cut by a shear. In that case, the lateral center of the trailing end of the preceding material to be rolled does not coincide with the lateral center of the leading end of the subsequent material, and the gap between the ends of the two materials is not uniform in the lateral direction. This makes the joining operation difficult. Camber is generated in the roughing roll for the following reasons. A difference between upper and lower torques is generated in the four-high mill stand when the bar is bitten in the four-high mill and a rolling torque is thereby generated. Consequently, the work rolls in the four-high mill stands are subjected to upper and lower opposite horizontal forces, and are thereby moved in a horizontal direction due to backlash between bearing boxes of the work rolls and a housing thereof, thus making the draft non-uniform in the lateral direction. As the thickness of the bar is reduced, the non-uniformity of the rolling reduction in the lateral direction is increased, thus increasing camber.
In the roughing train, when the mill stands are disposed separately at a long distance from each other, the length of the overall system is increased, and the material being rolled is thereby cooled excessively, which increases reduction in the temperature of the bars which occurs until the bars reach the joining position. Hence, the time required for heating and joining the bars is increased, and effective joining which utilizes the temperature of the bars is precluded. Also, as the length of the rolling system is increased, the installation cost is increased.
Furthermore, since the bar is rolled to a thickness of 30 to 40 mm in the roughing rolling, when the joining time is long, the joining device must travel a long distance to join the bars without using a looper. Assuming that the conventional joining time is about one minute, if the rolling rate at the exit of the finishing train is 600 m/min to obtain a 2 mm thick product while the rolling rate at the entrance of the finishing train is 30 m/min for bars having a thickness of 40 mm, the joining machine must travel 30 m. When a looper is used, a 30 m long looper is required, which is quite unpractical. Also, When the looper is used, since the bar makes contact with the rollers at the entrance of the looper and stops, it is locally cooled, deteriorating the quality thereof. Furthermore, since the bar having a thickness of 30 to 40 mm cools quickly, the temperature difference between the leading and trailing ends of the bar is large. This causes the temperatures to greatly differ from each other in portions between which the joined section is located, thus changing the finish temperature and deteriorating the quality of the product. Hence, it has been proposed in JP, A, 58-112601 to coil the bar at the exit of the roughing train and then to unroll the bar and join, as stated above. However, this system is complicated, and generation of flaws on the surface of the bar due to coiling and uncoiling must be prevented.